Heatbed Thermistor discrepancy

I currently have 3 printers running off of Duet boards, and one off of a RAMBO. All the Duet boards have Robotdigg Silicone heater, with the values:

M305 P0 L-8 H-8 T100000 B3950

The silicone is topped with an aluminium sheet, black BBQ paint, and glass from various suppliers. When one of the heaters reaches a 90c reading, it returns 90c with an IR heat gun. The other beds return this with a 120c reading.

The behaviour with layer adhesion at 90c on the errant heatbed is the same as the others at 120c. I print mostly in ABS and ASA, hence the high temperatures.

PLA is 55c and 65c respectively.

Is there a way to either adjust the target temperatures to reflect this in a slicer / firmware, or is there a likely different resistance value on the thermistor at play here?

First, unless your Duets are the older sort (Duet 06 or 085) then you shouldn't use the H and L parameters unless you have calibrated the ADC using 1% or better tolerance resistors. If you do need the H and L parameters then they will be different for each Duet.

If you are using Duet WiFi or Duet Ethernet then the differences might be due to the thermistors not being very accurate. If the reading at room temperature is accurate but the reading at high temperatures is not, then it's the B value that you need to adjust.

Note, when using a glass sheet on top of the bed, the surface temperature at 100C will be around 10C lower than the temperature of the aluminium. So it's normal to measure about 10C lower than the indicated temperature. Also, if the thermistor is embedded in the heater instead of being mounted directly on the aluminium, then there will also be a drop between the thermistor temperature and the aluminium temperature, perhaps another 10C, or even more when the heater is working hard. So the 120C reading may actually be the true thermistor temperature when the bed surface is at 90C.

Ah, yeah, so I have one 0.6 board on an old delta, and 2 wifis. That's probably a carry over from the firmware on that board.

I'll have a look at the reading between the two wifis at room temperature since they have the most similar physical build. With the silicone mats, the thermistor is in an insulated pouch, under the mat proper. It's possibly much more connected to the heater than with the kapton heater I have, but this doesn't seem to produce much of a real difference in readings.

I'll work on the B value and see if I can get it to correlate. It's just annoying having separate profiles in slic3r!

@mortarart try upping the B value to about 4010 I managed to get a Thermistor table from Keenovo and fed it thru a Spreadsheet that David sent me and for accuracy at 100 C I worked it out as 4008 for 120 it would need to be a little higher number but I don't have either the table or the Spreadsheet anymore after I lost my Windows machine to a crash

So in the end, I measured resistances at various temperatures, and while I'm not really sure how this works, the following gave me a B value of 3535k.

15c - 150000 resistance
70c - 210000 resistance

Now, the IR temperature measurement returns a lower number, and the heater is capable of topping 100c (firmware reading) which it was previously seemingly incapable of. There still seems to be about a 20c discrepancy left however, and I wonder if that's due to the thermistor placement. The other silicone mats, and the kapton mat all have a thermistor in the middle, while this one is within about 5cm of the edge of the bed.

Would lowering the B value further be advised, in order to get reads that reflect the same surface temperature?

Personally I accept what the firmware tells me, and I add whatever I find to be necessary to get good results. For example, I print PLA using an indicated bed temperature of 70C which in theory is too high (above the glass transition temperature of 55C). I am using a thermistor embedded within a silicone heater (which is not good if I want accurate temperature readings), and a 4mm thick glass bed (also not good).

Yeah fair enough. Adjusting the B value has resulted in reducing the temperature that the heatbed is actually outputting. That's a good enough solution for this for me. Hoping I won't have to do it with future printers.